Recent years have seen great effort being put in to actively develop thin displays based on light-emitting devices, such as organic EL (Electro Luminescence) devices and FEDs (Field Emission Devices).
It is known that in light-emitting devices the luminance of an element is proportional to the current density in that element. Such an element is regarded as having characteristics (e.g. applied voltage vs. current characteristics) which are so easy to vary that the luminance can be adjusted through voltage application only with difficulty. Presumably it is preferred if the element is driven using a constant current source.
For example, Japanese Unexamined Patent Application 10-319908/1998 (Tokukaihei 10-319908; published on Dec. 4, 1998, corresponding to U.S. Pat. No. 5,952,789; hereinafter, “Document 1”) discloses a technique to apply programmed current levels to organic EL elements (O-LED s) to cause the O-LED s to shine. FIG. 10 illustrates the structure of a pixel in an organic EL display (“pixel structure 100”) built based on the technique disclosed in the Application.
The pixel structure 100, as shown in FIG. 10, includes an O-LED 110, two transistors T1, T2, two data lines D1, D2, two select lines S1, S2 and a capacitor C1.
Each of the transistors has a source, gate, drain, and associated electrodes. The source electrode of the first transistor T1 is connected to the data line D1, and the source electrode of the second transistor T2 is connected to the data line D2. The gate electrode of the first transistor T1 is connected to the first select line S1, and the gate electrode of the second transistor T2 is connected to the second select line S2 via the capacitor C1. The drain electrode of the first transistor T1 is connected to the capacitor C1 and also to the gate electrode of the second transistor T2.
The combination of the data lines and the select lines enables the pixel structure 100 to operate in multiple modes including write select mode, write non-select mode, and light-emitting mode.
In write select mode, a predetermined current level (I1) is applied to the O-LED 110 as follows: The first transistor T1 conducts through the first select line S1, allowing the voltage on the first data line D1 to be applied to the gate of the second transistor T2 through the first transistor T1. As the voltage applied to the gate of the second transistor T2 increases, the second transistor T2 conducts and its internal impedance continuously decreases until the current through the second data line D2 reaches the current level I1.
In write select mode, a select signal sent through the second select line S2 stays HIGH. The second data line D2 is connected to the O-LED 110 through the second transistor T2. Therefore, the current level I1 reached flows through both the second transistor T2 and the O-LED 110.
If there exists a shift in the threshold voltage of the second transistor T2 or the transition voltage of the O-LED 110, the shift is accumulated across the capacitor C1 and compensated for by an increase or decrease in the voltage applied to the gate of the second transistor T2.
Thus, whatever shift exists in operating characteristics of either the O-LED 110 or the second transistor T2, or both, the shift hardly affects the current through the O-LED 110, hence the pixel luminance.
In write select mode, the select signal is HIGH on both select lines. In other words, the select signal on the first select line S1 becomes HIGH, causing the first transistor T1 to conduct. The select signal on the second select line S2 on the same row becomes HIGH (that is, write select mode), causing the second transistor T2 to conduct.
However, in write non-select mode, the select signal on the second select line S2 for all the other rows is made LOW (that is, write non-select mode). In other words, in write non-select mode, the second select line S2 is used to cause all the second transistors T2 on all the rows to which no data is written in the array not to conduct.
This is achievable, as shown in FIG. 10, by coupling the second select line S2 to an accumulation terminal through the capacitor C1. When the select signal on the second select line S2 is LOW, in write non-select mode, regardless of the potential accumulated across the capacitor C1, the gate of the second transistor T2 is adapted to receive a LOW signal so as to inhibit current from flowing through the second transistor T2 or the O-LED 110.
Therefore, the current detected along the second data line D2 flows only to selected O-LEDs 110, not to other pixels on that row.
In light-emitting mode, the first select line S1 is made LOW, thereby causing the first transistor T1 not to conduct. Simultaneously, the second select line S2 becomes HIGH. The combination of the HIGH potential on the second select line S2 and the potential stored across the capacitor C1 drives the gate of the second transistor T2 to that adjusted level. By doing this, the O-LED shines at its programmed current levels (that is, as programmed in write select mode) or luminance. In addition, in light-emitting mode, a constant control of the second data line D2 is carried out.
However, since it is difficult to actually assemble a constant current source drive circuit, in many cases a regulated current drive circuit is assembled around a constant voltage source. In such cases, a suggestion is made to provide a means which detects current in the element and to control so that the current detected by the detecting means becomes constant.
An example of an organic EL display which corrects luminance using such a current detecting means is disclosed by Japanese Unexamined Patent Application 2000-187467 (Tokukai 2000-187467; published on Jul. 4, 2000; hereinafter, “Document 2”). The display disclosed (hereinafter, “organic EL panel”) is of a passive matrix type including organic EL elements and has a structure shown in FIG. 11.
In FIG. 11, the organic EL panel 201 is made of a matrix of cathodes (C0 to Cn) and anodes (S0 to Sm), as well as organic EL elements located at their crossings and connected to a cathode drive circuit 202 driving the electrodes of the cathodes (C0 to Cn), an anode drive circuit (PG0 to PGm) 203 driving the electrodes of the anodes (S0 to Sn), and a current detecting circuits (IS0 to ISn) 204 detecting an output current from the anode drive circuit.
In other words, the organic EL panel 201 is configured to feed current values detected by the current detecting circuits 204 to a control device 205 so that ON times or ON currents of pixels are adjusted according to the detected currents.
Each current detecting circuit 204 is adapted, as shown in FIG. 12, so as to detect the voltage drop across a resistor (R1) 307 with an A/D converting circuit 306 for output.
Japanese Unexamined Patent Application 11-338561/1999 (Tokukaihei 11-338561; published on Dec. 10, 1999; hereinafter, “Document 3”) discloses a display of a passive matrix type having organic EL elements. The display has less current detecting means (current detecting circuits 204). An example of the structure of the passive matrix display is shown in FIG. 13.
Referring to FIG. 13, the passive matrix display has an organic EL panel 401 in which light-emitting elements Z11 to Znn are connected to the crossings of row electrodes R1 to Rn and column electrodes C1 to Cn.
Row drivers 421 to 42n driving the column electrodes C1 to Cn are connected to a current detect resistor Rd connected to a separate operating power source VB1 from the row electrodes R1 to Rn and sequentially addressed by selector circuits S11 to S1n. The column electrodes C1 to Cn in the matrix are connected to those terminals of the selector circuits S11 to S1n which are not connected to the current detect resistor Rd.
The voltage across the current detect resistor Rd is compared with a reference voltage Vref by a differential amplifier A1 and an error amplifier A2, inverted and amplified, and fed back to the inputs of constant current drive circuits 421 to 42n forming a row driver. Under these circumstances, the column electrodes C1 to Cn are sequentially connected to the current detect resistor Rd for current correction; the rows therefore do not need individual current detecting/correcting circuits, but can share a single, common circuit.
An example of an organic EL display which corrects luminance using such a current detecting means together is disclosed by Japanese Unexamined Patent Application 10-254410/1998 (Tokukaihei 10-254410; published on Sep. 25, 1998; hereinafter “Document 4”). The display disclosed is of an active matrix type including organic EL elements. FIG. 14 shows a block diagram of the active matrix display.
Referring to FIG. 14, the active matrix display includes an A/D converting circuit 511, computing circuit 512, frame memory 513, controller 514, scan circuit 515, write circuit 516, current circuit 517, current value memory 518, and display panel 519.
Still referring to FIG. 14, a luminance adjusting means drives all organic EL elements in the display panel 519 at a common, constant voltage, measures the current in each organic EL element, stores the measured current value in the current value memory 518, causes the computing circuit 512 to process that memory data and the display data externally fed through the A/D converting circuit 511, and adjusts the sum value of the currents through the pixels.
To achieve an active drive, each pixel in the display panel 519 has a structure illustrated in FIG. 15. Addressing a scan electrode line causes the FET 621 to conduct, storing the voltage on the data electrode line in the capacitor 623. Even when the FET 621 does not conduct, the FET 622 is controlled by way of the voltage across the capacitor 623 so as to adjust the current value through the organic EL 625.
Accordingly, the current detector 624 is placed between the FET 622 and the organic EL element 625. An A/D converting circuit 626 digitizes the output from the current detector 624 to produce digital data, which is stored in the current value memory 627 to adjust the sum of the current values.
However, in the passive matrix display disclosed in Document 2 (Tokukai 2000-187467), since the cathodes (C0 to Cn) are sequentially selected, the current through the organic EL element located at the crossing of the selected cathode (scan electrode line Ci) and the anode (signal electrode line Sj) can be measured by measuring the current through the anode (signal electrode line Sj). In the passive matrix display disclosed in Document 3 (Tokukaihei 11-338561), the current through the organic EL element can be measured by measuring the current through the associated column electrode (C1 to Cn).
However, in these passive matrix displays in Documents 2, 3, only those pixels which are connected to the currently selected electrodes shine, and the pixels do not shine in most of the non-select periods. Accordingly, to achieve a HIGH of overall luminance, the selected pixels must shine with extremely high luminance. For example, where the duty ratio is 1/100, an instantaneous luminance of 100×100=10000 cd/m2 is required in a select period to achieve a mean luminance of 100 cd/m2. Achieving such a high instantaneous luminance necessitates application of high voltage to the selected electrode, which is in general cases disadvantageous in terms of light emitting efficiency.
Meanwhile, the active matrix display disclosed in Document 1 (Tokukaihei 10-319908) goes through write select mode, write non-select mode, and then light-emitting mode, and therefore fails to produce expected luminance in a no-light-emitting period which inevitably occurs in a scan frame period, although the problem is not as serious as in the case of passive matrix displays.
In the active matrix display disclosed in Document 4 (Tokukaihei 10-254410), current flows through the organic EL element even when the associated scan electrode line is not being selected. Therefore, the display does not require as much instantaneous luminance as the passive matrix display. However, the aforementioned organic EL element current measuring method for passive matrix displays, that is, the collective current measurement for each signal lines in Document 2, does not work with active matrix displays.
Accordingly, in active matrix displays, current is measured for each pixel as shown in FIG. 15.
The illustrated arrangement, in which a separate current measuring means is provided for each pixel, has problems of a low TFT (Thin Film Transistor) integration in each pixel and a low aperture ratio of the panel due to the placement of the current measuring means with each pixel.